IP multicast is a mechanism for efficiently delivering IP content to end users (“clients”) from a source. IP multicast differs from unicast (one-to-one delivery) and broadcast (one-to-all delivery) in that it makes use of network based multicast routers (MCs) to distribute a single incoming IP stream to a set of clients and/or further MCs which belong to a given multicast group. IP multicast is considered an excellent technology for the delivery of bandwidth hungry services such as IP television (IPTV).
One of the key concepts of IP multicast is that of the multicast group address. Clients wishing to receive multicast content would join an IP multicast group via Internet Group Management Protocol (IGMP), in order to register with an upstream MC. MCs themselves generally use Protocol Independent Multicast (PIM) to set up a multicast distribution tree in a hierarchical structure. A sender can then simply send packets towards this IP multicast group address, so that all the clients registered in that group would receive those packets. In the multimedia domain, most of the time, multimedia content is framed in Real Time Protocol (RTP) packets, which run on top of User Datagram protocol (UDP) to be multicast delivered.
IP television (IPTV) is the name given to a range of services which allow television to be delivered over an IP network. Due to the flexible nature of an IP network, IPTV will allow for a much more personalised service to users, e.g. video-on-demand, with information delivered to users over unicast or multicast IP streams. Currently the predominant way of controlling these streams is to use the Real Time Streaming Protocol (RTSP), defined in IETF RFC 2326. RTSP does not specify a transport protocol but may be used, for example, to establish and control RTP media streams. RTSP is in many ways similar to the HTTP protocol used to request and exchange information over the Internet, but is tailored for streaming media such as audio and video. RTSP allows for a client to request particular media streams from a streaming server, and specifies commands such as PLAY and PAUSE. It does not deliver the continuous streams itself; instead, it controls the streaming servers for delivery.
Thus RTSP supports both unicast or multicast mode, with the selection depending on the network destination address. In unicast mode, the media is transmitted to a requested destination and port number chosen by the client. In multicast mode, server multicast data is transmitted towards a specific multicast address. The multicast IP address and port information could be decided by server or client.
For data delivery, most real-time media uses the RTP as a transport protocol, although RTSP is not tied to RTP. RTP provides end-to-end delivery services for streaming delivery with real-time characteristics, and consists of two parts. The RTP carries data with real-time characteristics, while the RTP Control Protocol (RTCP) monitors the quality of service and conveys information in an on-going session.
It is expected that users of mobile terminals such as mobile telephones will wish to avail themselves of IPTV services. Indeed, this is probably key to the business models of network operators currently installing high capacity cellular networks such as 3G networks. Mobile TV brings TV services to the mobile screen, but is more than traditional TV moved to a tiny screen. It provides the user with the freedom to watch TV content whenever and wherever he is. Besides watching live TV channels and video clips, Mobile TV also provides more interactive TV experience than traditional TV, such as voting and chatting. As the usage of mobile TV grows, cellular networks can be upgraded to meet mass-market demand.
Mobile TV is currently offered via streaming technology over point-to-point connections. In such situations, data packets are transmitted to a signal destination. Large-scale market deployment of Mobile TV will require new network capabilities of point-to-multipoint connections. This will require the support of both networks and mobile terminals. Using point-to-multipoint connections, data packets can be simultaneously transmitted to multiple destinations.
FIGS. 1 and 2 illustrate unicast (point-to-point) and multicast (point-to-multipoint) delivery of data packets of one TV channel in a cellular network. FIG. 1 is a schematic representation of elements of a network delivering a TV channel using unicast packets. A Mobile TV provider 101 sends packets across a backbone network towards a Gateway GPRS Support Node (GGSN) 102. The GGSN identifies the addresses and locations of users subscribed to the TV channel, and forwards the packets to various Serving GPRS Support Nodes (SGSN) 103, 104. The SGSNs 103, 104 route the packets through radio networks 105, 106, 107 towards end users using mobile terminals 111-118. Each end user requires its own stream of data, and therefore eight streams must be sent from the mobile TV provider 101, and four from the GGSN 102 to each SGSN 103, 104. Many streams of data are sent into each radio network 105-106. It will be appreciated that this results in very inefficient usage of network resources.
FIG. 2 is a schematic representation of elements of the same network delivering a TV channel using multicast packets. The backbone network 102, radio networks 105, 106, 107 must all support the use of multicast packets, and the gateway nodes must be able to route multicast packets. The mobile TV provider 101 need only transmit a single stream of multicast data packets. Each gateway node recognises that the packets are multicast, and forwards the same stream of data to more than one recipient. This ensures a more efficient use of network resources, but requires that all points in the network can deal with multicast data packets.
Within cellular networks, point-to-multipoint transmission is now essentially based on IP multicast. The introduction of the 3GPP Multimedia Broadcast/Multicast Service (MBMS) (3GPP TS 23.246) in GSM and UMTS cellular networks standardises the architecture and provides techniques for optimized transmission of a MBMS bearer service such as point-to-multipoint transmission, selective combining and transmission mode selection between Point-to-Multipoint (PTM) and Point-to-Point (PTP) bearers. It enables the efficient usage of radio-network and core-network resources, with an emphasis on radio interface efficiency.
In MBMS, the term “Broadcast” refers to the ability to delivery content to all users. In a cellular network, this means that a Mobile TV service would be broadcast to all mobile terminals in the cells of defined Service Area. Multicast, on the other hand, refers to services that are delivered only to users who have joined a particular multicast group. In a cellular network, this means that the service is delivered only to mobile terminals that have signed up. There is a clear need for MBMS to be introduced into Mobile TV.
In 3GPP, a logic component called the Broadcast-Multicast Service Centre (BM-SC) provides MBMS functionalities in the service layer for the Mobile TV provider. The BM-SC provides five main functionalities: membership; security; session and transmission; proxy and transport; and service announcement.
3GPP release 6 specifies that the BM-SC should send multicast IP packets towards a GGSN. However, in some cases, the GGSN may not support multicast operations. Therefore another option has been proposed in 3GPP release 7 to address this situation. IP multicast packets may be encapsulated inside IP unicast packets by the BM-SC for sending towards the GGSN. This is known as IP-in-IP and is described in IETF RFC 2003.
There is therefore a need to enable a media server to send multicast packets in a unicast tunnel dynamically. There is a further need to provide the control the encapsulation and recovery of IP multicast packets in unicast packets. There is a further need to control such encapsulation and recovery using the RTSP.